Recently, research and development of a lithium ion secondary battery has been progressing in which lithium ions transfer between a negative electrode and a positive electrode to perform charge and discharge as a battery having a high energy density. In particular, the nonaqueous electrolyte battery is expected to be a power source for hybrid automobiles or electric automobiles, or an uninterruptible power supply in a cellular phone base station, and it is required to have many properties in addition to increased energy density, which has hitherto been required, such as rapid charge and discharge property, long-term reliability, and safety.
Recently, a metal composite oxide has started to receive attention as a lithium host of an electrode. In particular, in an electrode material using a titanium oxide as a metal oxide, it is possible to stably perform rapid charge and discharge in terms of the potential properties thereof, and the electrode material has a property capable of giving a longer lifetime to the electrode compared to a carbon material conventionally used. Some electrode materials described above, however, have a defect of a low energy density, because titanium oxide has a higher potential to metal lithium than that of a generally used carbon electrode, and has a low capacity density per weight. For example, it is known that a lithium-titanium composite oxide such as Li4Ti5O12 has a theoretical capacity of about 175 mAh/g, which is lower than an electrode capacity of a generally used graphite electrode material of about 385 mAh/g. In many of these compounds, the number of equivalent sites in which lithium ions can be inserted is small due to the crystal structure thereof and the lithium is easily stabilized in the structure, and thus they have a defect in which a substantial capacity is decreased.
On the other hand, the electrode potential of titanium oxide is caused by an oxidation-reduction reaction between Ti3+ and Ti4+ when lithium is electrically inserted and extracted. The electrode potential of titanium oxide, thus, is electrochemically limited to a potential of about 1.5 V on the basis of the metal lithium. In order to further improve the energy density, accordingly, it is necessary to improve the electrode capacity of the material.
In view of the circumstances described above, a monoclinic titanium dioxide, TiO2 (B), has recently received attention. In a spinel type lithium titanate Li4Ti5O12, which has been practically used, the number of lithium ions capable of deinsertion is 3 per unit chemical formula, and thus the number of lithium ions capable of deinsertion is ⅗ per titanium ion, i.e., theoretically at most 0.6. On the other hand, in TiO2 (B), the number of lithium ions capable of deinsertion is at most 1.0 per titanium ion, and thus the theoretical capacity is high, i.e., about 330 mAh/g. TiO2 (B), accordingly, can be expected to be an electrode material having a high capacity.
On the other hand, many of the monoclinic titanium dioxide compounds have a property as a solid catalyst, and thus it is known that the compounds have a high reactivity with an organic electrolytic solution when they are used as a battery electrode material. When titanium oxide is reacted with the electrolytic solution, many problems such as decreased properties of the electrode caused by a reaction by-product, an increased internal resistance of the battery, and a decreased life performance caused by a deteriorated electrolytic solution occur. In particular, if there is a slight amount of water contained in production steps of starting materials and an assembly step of a battery, the titanium oxide has solid acid points having a high reactivity on the surface thereof. A monoclinic titanium dioxide compound, TiO2 (B), expresses especially a high solid acidity in water. It is difficult to chemically complete remove water from the electrode material for a battery in terms of the properties of the starting materials and the cost.
In view of these problems, measures in which the surface on which the solid acid points (active points) such as hydroxyl groups (OH−) and hydroxyl radicals (OH.) exist of TiO2 (B) is modified are adopted. As such measurements, for example, methods of modifying an alkali metal cation (Li+, Na+ or K+), an alkaline earth metal cation (Mg+ or Ca+), a transition metal (Mn3+, Co2+ or Cu+) and a sulfide ion (S2−), or a sulfate ion (SO4−) are disclosed. The methods, however, have a problem in which the modified cation can be eluted into the electrolytic solution during long time use.